With speedup in access line, a demand for voice and/or video streaming service is increasing. Currently used streaming service employs a unicast system which transmits from a voice/video server, data packets respectively dedicated to multiple subscriber terminals. Therefore, the voice/video server is forced to transmit a large number of packets, causing problems such that data transmission load is high in the voice/video server, as well as increasing a load on a relay network which relays the large number of packets.
In view of the situation above, telecommunication carriers, such as CDSP (content delivery service provider), and ISP (Internet Service Provider), now consider employing multicast system for the voice/video streaming service. In the multicast system, the voice/video server transmits only one data packet to multiple subscriber terminals, and a packet relay device within a relay network copies the data packet as appropriate, thereby executing a packet transfer to the multiple subscriber terminals. Therefore, the number of packets transmitted by the voice/video server is suppressed to a small number, and the load onto the server can be reduced. Furthermore, the number of packets flowing in the relay network is also suppressed to a small number and the load onto the relay network can be reduced as well.
In the meantime, the telecommunication carriers may construct a network employing a virtual router (hereinafter, referred to as “VR”), for the purpose of reducing a cost in building the network or separating traffic among subscribers. VR is a technique to configure a function of multiple virtual routers within one packet transfer device. Since such function of multiple routers can be implemented with one packet transfer device, it is possible to reduce the number of installed routers in the circumstance where multiple routers are required at the same spot.
For example, as for multiple ISPs, ADSL (Asynchronous Digital Subscriber Line) line service provider or FTTH (Fiber To The Home) line service provider, for providing an access network to connect the ISPs and the subscribers, may be capable of providing a function of routers dedicated to the ISPs, respectively. Alternatively, a telecommunication carrier who offers a wide area IP network service called as IP-VAN (IP Virtual Private Network) allocates a VR to each of customers, thereby providing the VPN service without interference in traffic among customers and establishing communication within each customer only.
FIG. 2 shows a configuration of a conventional packet relay device in a form of functional block of VR, with which a network adapted for multicast transfer can be constructed, as well as VR function is being installed therein. The packet relay device as shown in FIG. 2 implements VR 61a and VR 61b. Each of the VRs is provided with UPLINK information 611a, 611b, multicast routing tables 612a, 612b, PIM 613a, 613b, IGMP PROXY 614a, 614b, and the like. Reference numerals 12a to 12d denote subscriber terminals, and they are connected to the packet relay device 6 via communication lines. Subscriber terminals requesting to participate in a multicast delivery target group transmit IGMP Report messages 13a to 13d to the packet relay device 6.
The packet transfer device which has received the IGMP Report messages refers to Uplink information using as a key a multicast group address included in each of the IGMP Report messages, selects an interface connected to a line being upstream of the multicast, and transmits to the upstream router a PIM protocol message to allow the terminal to participate in the multicast group. In FIG. 2, VR 61a and VR 61b respectively receive IGMP Report messages 13a, 13b and 13c, 13d, and transmit PIM protocol messages 13e and 13f to the upstream routers.
The multicast system has a function to reduce the load on the relay network, but there is a possibility the load on the relay network is increased if the multicast system is employed in the network utilizing a VR. In order to clarify a problem to be solved by the present invention, as a reference example, an example in which a network is constructed using the packet relay device as shown in FIG. 2 and data delivery in the network is preformed through multicast system will be described with reference to FIG. 3. In FIG. 3, multicast traffic 32 comprising data packets is delivered from the multicast server 3, via ISP 41a, router 2a, VR 61a and VR 61b, and via SP 41b, router 2b, VR 61c and VR 61d, respectively to the subscriber terminals 12a to 12d and 12e to 12h. Here, VR 61a and VR 61b are located within the packet relay device 6a, and VR 61c and VR 61d are located within the packet relay device 6b. 
A multicast packet is copied by a router on a path from the multicast server to the subscriber terminal, and then the copy is delivered to the subscriber terminal. In FIG. 3, a data packet from the multicast server 3 is copied by a router (not illustrated) in the Internet 4 and delivered to the routers 2a and 2b. Subsequently, the copied data packets are delivered to VR 61a and VR 61c from the router 2a, and another copied data packets are delivered to VR 61b and VR 61d from the router 2b. 
In the multicast delivery, it is desirable to carry out copying in a router located as close as possible to a subscriber, thereby reducing the number of data packets transferred within the network and also reducing the load onto the relay network. However, in the example of FIG. 3, the data packets belonging to the identical multicast delivery (packets having the same destination address and data) are redundantly delivered from the router 2a and the router 2b, to the VR 61a and VR 61b in the packet relay device 6a, and to the VR 61c and VR 61d in the packet relay device 6b. 
The situation above occurs since the VRs with conventional function have to operate independently even if they are located within the same packet relay device, and the multicast traffic cannot be shared between the VRs. Therefore, as it is shown in FIG. 3, the router 2a and the router 2b have to transmit the multicast traffic to all the VRs within the packet relay devices 6a and 6b, increasing the load onto the relay network.